The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly.
The anticipated widespread deployment of third and fourth generation networks has led to the parallel development of a number of new infrastructure architectures involving a variety of classes of devices, of wireless access point units and of applications which may require different data rates, coverage areas or transmission powers. Unlike a conventional third or fourth generation communications terminal such as a smartphone, an MTC-type terminal is preferably relatively simple and inexpensive, having a reduced capability. Examples of recent developments include so-called machine type communication (MTC) applications, which are typified by semi-autonomous or autonomous wireless communication devices (i.e. MTC devices) communicating small amounts of data on a relatively infrequent basis. Examples include so-called smart meters which, for example, are located in a customer's house and periodically transmit information back to a central MTC server data relating to the customers consumption of a utility such as gas, water, electricity and so on. Other examples include relay nodes which provide assistance to local terminal communicating with a base station.
Whilst it can be convenient to have different systems addressing different needs from different mobile network users, the additions of new infrastructure and new services can also create an infrastructure problem, which is not desirable in a mobile network.
With the continuous growth in data transmitted in mobile networks, continually increasing network capacity comparatively is a problem faced by the industry. There are three parameters which can be changed in order to increase Radio Access network capacity: higher spectral efficiency, more radio spectrum and denser cell layout. The two former of these have limitations on the expected gains over today's LTE, and certainly improvements on the order of magnitude or more are not possible. Thus, in order to meet the stated 1000× capacity targets, small cells are getting a lot of attention [1].
However, although the coverage and capacity of fourth generation networks is expected to significantly exceed those of previous generations of communications networks, there are still limitations on network capacity and the geographical areas that can be served by such networks. These limitations may, for example, be particularly relevant in situations in which networks are experiencing high load and high-data rate communications between communications terminals, or when communications between communications terminals are required but the communications terminals may not be within the coverage area of a network. In order to address these limitations, in LTE release-12 the ability for LTE communications terminals to perform device-to-device (D2D) communications will be introduced.
D2D communications allow communications terminals that are in close proximity to communicate directly with each other, both when within and when outside of a coverage area or when the network fails. This D2D communications ability can allow user data to be more efficiently communicated between communications terminals by obviating the need for user data to be relayed by a network entity such as a base station, and also allows communications terminals that are in close proximity to communicate with one another although they may not be within the coverage area of a network. The ability for communications terminals to operate both inside and outside of coverage areas makes LTE systems that incorporate D2D capabilities well suited to applications such as public safety communications, for example. Public safety communications require a high degree of robustness whereby devices can continue to communicate with one another in congested networks and when outside a coverage area.
Fourth generation networks have therefore been proposed as a cost effective solution for public safety communications compared to dedicated systems such as TETRA which are currently used throughout the world. However, the potential coexistence of conventional LTE communications and D2D communications within a single coverage area or network may increase the complexity of coordinating communications and resource allocation within an LTE network, and may also lead to potential compatibility issues between conventional and D2D capable LTE communications terminals.
It has been envisaged therefore that there is a requirement to perform D2D communications between a group of communications terminals which are outside of a coverage area of an LTE mobile communications network, which may communicate between devices using a repeat request communications protocol or the like.